Process for milling long workpieces

ABSTRACT

A distorted workpiece is initially held in a position in which it has a free sag near the center of its length. The distorted workpiece is subsequently supported near the center of its length so as to eliminate only the elastic deformation of the workpiece which is due to the free sag.

This invention relates to a process of milling long workpieces,particularly crankshafts, wherein the workpiece is supported near thecenter of its length as it is machined, the workpiece being roughendedin a preliminary machining operation and finish-machined in a succeedingmachining operation.

A workpiece to be machined first at the ends, particularly a longworkpiece, must be supported near the center of its length so that theworkpiece can be centered and vibration can be avoided. This isparticularly important when small tolerances are required, as is thecase with crankshafts and camshafts. As such workpieces are supported bybackrests or supporting rollers, the workpiece must be provided by aseparate turning operation on a lathe with a rib for engaging thebackrest so that the workpiece can be properly centered. Largecrankshafts having a length of 2 to 3 meters or more must be machined intwo operations at the mainshaft portions and at the crankpins beforethey can be ground. The first of these operations consists of apreliminary roughing operation and the second operation consists of asucceeding finish-machining operation. The roughing operation causesinherent stresses in the material to be released so that the crankshaftor other workpiece will be distorted. Conventionally, the workpiece issupported by a centering backrest which engages the workpiece at the ribprovided for this purpose or at a previously milled mainshaft portion,and the workpiece will be traightened by the backrest and will bemachined in this straight condition. But when the backrest issubsequently opened, the crankshaft or other workpiece will spring backto the distorted position and must then be straightened in a complicatedoperation by a press. That straightening operation is very expensivebecause it cannot be automatically controlled. When such a straighteningoperation cannot be performed, the grinding allowance must be so largethat the distortion of the shaft is eliminated by grinding. But theremoval of such allowances by grinding is very time-consuming andrequires a grinding machine which meets very high requirements.

For this reason it is an object of the invention to eliminate thesedisadvantages and to provide a process for milling long workpieces,particularly crankshafts, wherein a turning operation for providing arib for engagement by a backrest is not required, and even though astraightening operation is not performed, the milled workpiece will bevirtually undistorted, and the workpiece can be machined to particularlysmall tolerances.

In the process according to the invention, this object is accomplishedin that the workpiece to be machined is supported in such a manner thatany plastic deformation of the workpiece which is due to the release ofinternal stresses in the material as a result of a preceding machiningstep is maintained and the supporting means are used only to eliminatethe elastic sag of the workpiece at the point where it is supported,which sag is ascertained empirically and/or by calculation.

Because the workpiece is no longer clamped to center the workpiece, theneed for a turning operation for forming the workpiece with a rib forengagement by the backrest is eliminated because a mainshaft portionwhich has not yet been machined is sufficient for the elimination of thefree sag, which is due to the weight of the workpiece. Any occurringinaccuracies are not significant because they are eliminated by theremoval of material during the succeeding milling operation. This isparticularly true for the finish-machining operation, during which thesupport does not raise the workpiece to the center of the machine anddoes not center the workpiece but again eliminates only the free sag,i.e., that portion of the distortion which is due to the weight of theworkpiece, whereas the curvature which has been caused by the inherentstresses during the roughing operation and any downward deflection willbe maintained. When only the free sag, which is due to the weight of theworkpiece, has been eliminated, the workpiece is held in a curvedposition and is machined in that position. As a result, the entiredistortion resulting from the roughing operation is eliminated by theremoval of material during the finish-machining operation. As thefinish-milling operation removes only a small amount of material so thatadditional stresses are hardly released, the crankshaft or otherworkpiece will not exhibit a substantial further distortion and closetolerances will be ensured. Besides, there is no need to straighten theworkpiece before the grinding operation, the grinding allowance can beminimized and the grinding time is greatly shortened. The processaccording to the invention may be used for milling operations in whichthe workpiece is stationary or rotated because it is sufficient tosupport the workpiece so as to eliminate the free sag which is due tothe weight of the workpiece. This can be effected when the workpiece isstationary or rotating.

This process can be carried out in a simple, efficient manner by asuitable milling machine which is provided with a backrest which maycomprise two individually adjustable gripping jaws, one of whichconstitutes a supporting jaw adapted to the vertically raised intoengagement with the workpiece, whereas the other jaw constitutes aretaining jaw engageable with the workpiece from above to clamp theworkpiece against the supporting jaw. The supporting jaw is providedwith a detector for indicating the position of initial contact of thesupporting jaw with the workpiece, a positioning drive is associatedwith the supporting jaw and operable to raise the supporting jaw fromthat position of initial contact to a preselectable extent, and theretaining jaw is adapted to cooperate with the raised supporting jaw.With the aid of such backrest, the sag of the shaft, which sag can bedetermined empirically or by calculation, can be eliminated withoutdifficulty because this can be accomplished by imparting to thesupporting jaw an additional upward movement which corresponds to theextent of said sag. The additional upward movement can be initiated inany desired manner, e.g., by hand, by means of cams, decade switches,punched tape and the like. When the selected additional upward movementhas been performed by the supporting jaw, the latter is locked inposition. Thereafter, the retaining jaw--it will be understood that twoor more retaining jaws may be provided--must be engaged with theworkpiece to clamp the latter against the supporting jaw. It is apparentthat such a backrest can be operated fully automatically. Besides, thebackrest can grip directly the as-forged mainshaft portion so that it isno longer necessary to form the workpiece before the preliminarymachining operation with a rib for engagement by the backrest by aturning operation.

Because it is important to determine the position of the point ofcontact between the jaw and the supporting jaw and the workpieceaccurately, the detector comprises in accordance with the invention afeeler which protrudes from the supporting surface of the supporting jawand is adapted to be depressed against a spring bias to trigger acontrol signal when the sensor is flush with the supporting surface. Asthe supporting jaw is moved toward the workpiece, that feeler isdepressed into the supporting surface of the supporting jaw against thespring bias and will be exactly flush with the supporting surface whenthe latter contacts the workpiece. As soon as this position has beenreached, a suitable signal generator generates a signal, which indicatesthat the movement of the supporting jaw must now be continued for adistance corresponding to the free sag.

It will also be desirable to provide the drives for positioning the jawswith brakes or to provide the jaws themselves with locking means. Bymeans of the brakes for the positioning drives, the displacement of thejaws can be exactly controlled and the jaws can be locked in any desiredposition. It will be understood that the latter effect can also beobtained by separate means for clamping the jaws. When the jaws havethus been locked, the workpiece will be clamped in the desired positionfor the succeeding machining operation without a loading of thepositioning drives.

When it is desired to support a workpiece which is rotated as it ismachined, the gripping jaws and their positioning drives are mounted ina carrying ring which surrounds the workpiece and is rotatably mountedand adapted to be arrested in datum rotational position, in which thesupporting jaw extends vertically upwardly. The carrying ring can easilybe moved to said datum rotational position under the control of controlswitches. When the carrying ring is in said position, the supporting jawcan be engaged with the workpiece from underneath, as is desired, andcan force the workpiece upwardly so as to eliminate the free sag. Whenthe workpiece has been fixed in this position by the retaining jaw, thecarrying ring can then be rotated during the machining of the workpiecewhereas the gripping is maintained.

Embodiments of the invention are shown by way of example on theaccompanying drawings, in which

FIG. 1 is an end elevation showing a backrest according to the inventionfor use in a milling machine for machining a workpiece while the latteris stationary and

FIGS. 2 and 3 are, respectively, a transverse sectional view and an endelevation showing a backrest for use in a milling machine for machininga workpiece while the latter is rotated.

The backrest 1 shown in FIG. 1 comprises a supporting jaw which isvertically guided and is adapted to be engaged with a workpiece 3 fromunderneath. The drive 4 for positioning supporting jaw 2 cooperates witha wedge mechanism 5 and is operable to lift the jaw 2 to a preselectableextent above the position in which the jaw 2 has initially contacted theworkpiece. The position in which the supporting jaw 2 initially contactsthe workpiece is indicated by a detector 6 which comprises a feeler 7biased by a spring 8 to protrude above the supporting surface 2a of thesupporting jaw 2 to cause a pulse generator 9 to generate a signal assoon as the feeler 7 is flush with the supporting surface 2a, whichmeans that the supporting jaw 2 is in the position of initial contactwith the workpiece. The movement of the supporting jaw 2 is thencontinued for a predetermined extent above said position of initialcontact. That extent equals the free sag of the workpiece at the pointwhere it is supported. The displacement of the supporting jaw 2 can bechecked by a displacement-measuring instrument 10.

Two retaining jaws 11 cooperate with the supporting jaw 2 to clamp theworkpiece against the supporting jaw 2. A separate positioning drive 12is associated with each retaining jaw 11. To permit the jaws to belocked in their gripping position, the retaining jaws 11 are providedwith locking means 13 and the drive 4 for positioning the supporting jaw2 comprises a brake 14.

To eliminate the need for a turning operation for forming a rib forengagement by the backrest and to ensure that the distortion of theworkpiece due to the release of inherent stresses during the preliminarymachining operation can be eliminated by the succeeding machiningoperation, the workpiece 3 which is to be machined or which has beensubjected to the preliminary machining operation and is to be machinedfurther is supported by the backrest 1. The latter engages the rawmainshaft portion and acts to eliminate only the free sag of theworkpiece whereas the inaccuracy which is due to the absence of a ribengageable by the backrest or the distortion that is due to thepreliminary machining operation is maintained. For this purpose thesupporting jaw 2 of the backrest 1 is moved toward the workpiece 3 frombelow to the position of initial contact of the supporting jaw with theworkpiece; this position is indicated in solid lines in FIG. 1. Fromthis position of initial contact, the supporting jaw 2 is raised furtherso as to raise the workpiece 3 by a distance f, which corresponds to thefree sag but does not take into account the curvature that is due to thepreliminary machining operation and is indicated by the eccentricitiese₁, e₂. The supporting jaw 2 is fixed in the resulting position by thebrake 14. The workpiece 3 is then held clamped by the retaining jaws 11(position shown in dotted lines in FIG. 1). This clamping of the shaftin a position in which only the sag of the shaft is eliminated whereasthe shaft is still a distorted state will eliminate the need for aturning operation to form the shaft with a rib that is engageable by thebackrest and permits the undesired distortion to be eliminated by thesucceeding machining step. As a result, the workpiece need not bestraightened before the subsequent grinding operation and closetolerances can be adhered to without a need for a large grindingallowance. The extent of the sag is ascertained empirically or bycalculation and provides a preselected extent by which the supportingjaw 2 must be raised from the position of initial contact of thesupporting jaw with the workpiece. The raising movement can be monitoredby the displacement-measuring instrument 10.

When it is desired to machine a workpiece 3 as it is rotated, a backresthaving rotatable gripping jaws must be used. Such backrest 15 is shownin FIG. 2 with open jaws and in FIG. 3 with the jaws in grippingposition. In this case a supporting jaw 16 and a retaining jaw 17 aremounted in a carrying ring 18 and extend radially therein and areadjustable by means of a screw 19, bevel gears 20 and an electric motor22, which is adapted to be fed via slip ring contacts 21. The supportingjaw 16 is again provided with a detector 23 for indicating the positionof initial contact between the jaw 16 and the workpiece 3. Adisplacement-measuring instrument 24 serves to monitor the displacementof the jaw. The carrying ring 18 is rotatably mounted in the backrest 15so that the jaws 16, 17 which grip the workpiece 3 can rotated with theworkpiece as it is machined. In order to support the workpiece 3 in aposition in which the free sag is eliminated but the distortion of theworkpiece is maintained, a datum rotational position is defined for thecarrying ring 18. In that datum position, the supporting jaw 16 isadapted to be vertically raised into engagement with the workpiece andto force the workpiece upwardly to an extent that is equal to the freesag. When the supporting jaw 16 has reached its final position, theretaining jaw 17 is lowered to grip the workpiece 3. For a machining ofthe workpiece 3 as it is rotated, the carrying ring 18 can now berotated by means of a drive gear 25 and a pinion 26, and the grippingjaws 16, 17 and the drives for positioning them will follow thatrotation.

What is claimed is:
 1. In a process of subjecting a long workpiece to apreliminary rough-milling operation causing distortion of the longworkpiece by a plastic deformation and a subsequent finish-millingoperation while supporting the workpiece near the center of its length,the improvement of(a) initially holding the rough-milled workpiece in aposition wherein it has a free sag near the center of its length, thefree sag causing an elastic deformation of the long workpiece (b)determining the extent of the free sag, and (c) supporting the workpiecenear the center of its length sufficiently to eliminate solely theelastic deformation and without eliminating the distortion of the longworkpiece while finish-milling the workpiece.
 2. In the process of claim1, the workpiece being a crankshaft.
 3. In the process of claim 1 or 2,the rough-milling causing internal stresses in the material of theworkpiece to be released to cause the distortion by plastic deformation.4. In the process of claim 1, 2 or 3, the extent of the free sag beingdetermined empirically.
 5. In the process of claim 1, 2 or 3, the extentof the free sag being determined by calculation.